From 80e1e823989ec44d8e35bdfddadbddcffec90424 Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Sun, 7 Feb 2010 10:11:23 -0800 Subject: Fix race in tty_fasync() properly MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit This reverts commit 703625118069 ("tty: fix race in tty_fasync") and commit b04da8bfdfbb ("fnctl: f_modown should call write_lock_irqsave/ restore") that tried to fix up some of the fallout but was incomplete. It turns out that we really cannot hold 'tty->ctrl_lock' over calling __f_setown, because not only did that cause problems with interrupt disables (which the second commit fixed), it also causes a potential ABBA deadlock due to lock ordering. Thanks to Tetsuo Handa for following up on the issue, and running lockdep to show the problem. It goes roughly like this: - f_getown gets filp->f_owner.lock for reading without interrupts disabled, so an interrupt that happens while that lock is held can cause a lockdep chain from f_owner.lock -> sighand->siglock. - at the same time, the tty->ctrl_lock -> f_owner.lock chain that commit 703625118069 introduced, together with the pre-existing sighand->siglock -> tty->ctrl_lock chain means that we have a lock dependency the other way too. So instead of extending tty->ctrl_lock over the whole __f_setown() call, we now just take a reference to the 'pid' structure while holding the lock, and then release it after having done the __f_setown. That still guarantees that 'struct pid' won't go away from under us, which is all we really ever needed. Reported-and-tested-by: Tetsuo Handa Acked-by: Greg Kroah-Hartman Acked-by: Américo Wang Cc: stable@kernel.org Signed-off-by: Linus Torvalds --- drivers/char/tty_io.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) (limited to 'drivers/char') diff --git a/drivers/char/tty_io.c b/drivers/char/tty_io.c index c6f3b48be9dd..dcb9083ecde0 100644 --- a/drivers/char/tty_io.c +++ b/drivers/char/tty_io.c @@ -1951,8 +1951,10 @@ static int tty_fasync(int fd, struct file *filp, int on) pid = task_pid(current); type = PIDTYPE_PID; } - retval = __f_setown(filp, pid, type, 0); + get_pid(pid); spin_unlock_irqrestore(&tty->ctrl_lock, flags); + retval = __f_setown(filp, pid, type, 0); + put_pid(pid); if (retval) goto out; } else { -- cgit v1.2.3